The first sizable array of vertical-cavity, surface-emitting lasers was disclosed by Jewell et al. in U.S. Pat. No. 4,949,350. They first grew a vertical-cavity Fabry-Perot resonator structure on a substrate. The initial structure was laterally undefined. Vertically, it consisted of upper and lower interference mirrors separated by an optical distance equal to the lasing wavelength. The mirrors sandwiched an active layer of several quantum well layers, which emitted light at the lasing wavelength when current was passed through them. All the layers were III-V semiconductors epitaxially deposited by molecular beam epitaxy on the conductive GaAs substrate. The layers above the active region were p-type, while those below were n-type so as to form a laser diode. A layer of gold was then deposited over the upper mirror.
The laser array was then laterally defined by a photolithographic definition of a nickel mask above the intended lasers followed by chemically assisted, ion-beam etching. The ion-beam etching was carried through the entire vertical-cavity structure so as to create an array of pillars having heights of more than 5 .mu.m. Each pillar was a separate laser and was electrically selected by contacting the metal at the top of the respective pillar. The conductive substrate served as a common counter electrode. Light was emitted through the substrate. Jewell et al. demonstrated their invention with lasers having diameters ranging down to 2 .mu.m. Thus, it became possible to fabricate extremely dense arrays of lasers.
The pillar lasers of Jewell et al. suffer from several problems. Electrical contact needs to be made to the top of the high aspect-ratio pillars. For small pillar lasers, the relatively large sidewalls cause excessive recombination. Heat cannot be efficiently dissipated in the pillar structure. The sophisticated processing of Jewell et al. raises questions of manufacturability. They suggested planarization with polyimide which would maintain the index-guide optical waveguiding function and current confining function of the previously defined pillars and ease the contacting problem. Work is progressing on this approach and on regrowth with insulating AlGaAs, which would help solve the recombination and thermal dissipation problems, but the results are not totally satisfactory.
Orenstein et al. have disclosed a planarized array of vertical-cavity, surface-emitting lasers in U.S. patent application, Ser. No. 480,117, filed Feb. 14, 1990, in "Lateral definition of high performance surface emitting lasers by planarity preserving ion implantation processes," Conference Proceedings, CLEO, May 21-25, 1990, pages 504-505 and in "Vertical-cavity surface-emitting InGaAs/GaAs lasers with planar lateral definition," Applied Physics Letters, volume 56, 1990, pages 2384-2386. They grew the same vertical-cavity structure as in the Jewell et al. array. However, they performed the lateral definition by ion implanting protons in regions surrounding the intended lasers and extending down to just above the active layer. The protons reduced the conductivity of the implanted region. Thereby, current was guided through the laser area. Thus Orenstein et al. retained the current guiding of Jewell et al. but sacrificed their index guiding since the protons did not have a significant effect on the refractive index. The deep ion implantation of their technique places a lower limit on the size of the lasers and the separation between lasers.
Although vertical-cavity, surface-emitting lasers provide the advantage of lasers of very small area and low threshold current, some applications require high optical power. In principle, a surface-emitting laser can achieve high-power by a simple increase in the cross-section of the lasing region with a constant current density. We have performed experiments that have demonstrated that this technique works only poorly. For larger sized surface-emitting lasers, the produced laser light is filamented into irregularly shaped and perhaps separated lasing areas. Similar filamentation has been observed in edge-emitting diode lasers due to inhomogeneities in the gain and refractive index distributions of the optical waveguides. For vertical-cavity diode lasers, filamentation may additionally arise from spatial variations in mirror reflectivities, which are above 99% because of the short gain length and high cavity finesse. The spatial variations are enough to induce lasing preferentially in some regions but not in others. A side from efficiency and thermal problems, the sparsely connected filaments are not likely to be phase-locked or even to have precisely the same frequency. That is, a large area surface-emitting laser tends to lose its laser characteristics. Furthermore, even medium sized lasers (.about.5-40 .mu.m in diameter) are bound to oscillate in a large number of modes, the distribution of which is uncontrollable. Some applications need a single mode of high-power lasing light.
Yoo et al. have disclosed an array of small, phase-locked lasers in "Fabrication of a two-dimensional phased array of vertical-cavity surface-emitting lasers," Applied Physics Letters, volume 56, 1990, pages 1198-1200. In a refinement of the Jewell et al. technique, they fabricated a rectangular array of more than 160 lasers formed within a 25 .mu.m circle. Each laser element had a square size of 1.3 .mu.m and was separated from neighboring laser elements by less than 0.1 .mu.m. The circular array was planarized with polyimide and a common upper electrode attached to all the lasers. The angular distribution of the far-field optical intensity showed substantial, though possibly not complete, phase locking between the lasers. Yoo et al. were able to achieve phase-locking between the strongly waveguiding pillars of Jewell et al. only by the very small separation between pillars and the small areas of the pillars. The calculations of Yoo et al. in "Array Mode Analysis of Two-Dimensional Phased Arrays of Vertical Cavity Surface Emitting Lasers," IEEE Journal of Quantum Electronics, volume 26, 1990, pages 1039-1051 have shown this requirement of small laser spacing for strongly waveguided structures. However, such a structure and associated processing produce very high surface recombination on the sides of the pillars because of the large surface-to-volume ratio. As a result, their phase-locked array showed poor efficiency and threshold current, and their phase locking was not complete.
Deppe et al. have disclosed another phase-locked surface-emitting laser array in "Phase-coupled two-dimensional Al.sub.x Ga.sub.1-x As-GaAs vertical-cavity surface-emitting laser array," Applied Physics Letters, volume 56, 1990, pages 2089-2091. They stopped the epitaxial growth of the vertical cavity with the upper spacer. They then formed a 2 .mu.m wide Mn-Al metallization grid on top of the upper spacer and an insulating dielectric stack on top of the grid-covered spacer. Lasing did not occur beneath the grid.
Phase-locked arrays of lasers present several unique applications. If the laser elements are phase locked with non-zero phase differences, the far-field intensity assumes a multi-lobed or at least off-axis pattern with the details of the patterns depending on the number of elements and the relative phase differences between the elements. If the phase differences are controlled, then the intensity pattern can be controlled.